Introduction
Obstetric ultrasonography has grown in popularity fast over the last decade and is now a standard component of antenatal care. Its safety and non-invasive nature make it acceptable to almost all, and it is used as a modern diagnostic and monitoring aid . It is also helpful as a tool to help take timely decisions and a guide for appropriate clinical interventions to reduce maternal and perinatal morbidity and mortality. The routine use of ultrasonography in the first and second trimester for screening for fetal anomalies is well-established and is a common obstetric practise worldwide.However its use on the labour ward has still not been fully appreciated although the indications for its use are growing rapidly. The use of ultrasound on labour ward was recently studied prospectively in a 4 month period in the U.S involving 2679 admissions to the labour ward, including 1363 non-labouring patients1. Almost 1 in 4 women admitted to labour ward required ultrasound scanning. Of those, approximately 68% were performed in non-labouring patients. The indications for ultrasound scanning in non-labouring women were mainly: amniotic fluid volume index in 15.8%, SROM in 15.6%, postdate in 9.8%, placental location in 9.6% and decreased fetal movement in 9.3%. The indications in labouring women were fetal presentation in 34.4%, confirmation of vertex presentation in 20.3%, preterm labour in 12%, multiple gestations in 7.3% and malpresentation in 7.3%. Overall, ultrasound scanning was performed in 15% of women in labour and 31% of non-labouring women. This study highlighted the importance of ultrasound as a diagnostic tool in the labour ward setting.
Ultrasound scanning on labour ward is used for various clinical indications as listed below:
- Preterm labour
- Unbooked pregnancies
- Preterm Premature Rupture of Membranes (PPROM), Spontaneous Rupture of Membranes (SROM)
- External Cephalic Version
- Fetal well-being
- Fetal viability
- Antepartum haemorrhage
- Multiple pregnancies
- Induction of labour
- Labour and Instrumental Delivery
- Prior to Caesarean section
- Post-partum period
- Anaesthesia related procedures
- Research
Preterm Labour
Spontaneous preterm delivery occurs before 37 weeks gestation in about 10% of pregnancies and is a major cause of perinatal morbidity and mortality. Diagnosis of preterm labour is crucial as it indicates the need to take further measures to improve the neonatal outcome such as hospitalisation, administration of steroid injections, use of tocolytics and in-utero transfer, if appropriate. However only a minority of pregnant women admitted with threatened preterm labour with regular contractions2, deliver before term.
A short cervical length and a raised cervical-vaginal fetal fibronectin concentration are shown to be the strongest predictors of diagnosing preterm birth. Cervical length measurement by transvaginal scanning (TVS) is shown to be more sensitive than the ‘traditional vaginal’ examination in the prediction of preterm birth. The most common cut off for cervical length is between 15 and 30 mm. A cut-off that is close to 30 mm has a high sensitivity with high false-positives in contrast to a cut-off close to 15 mm, which has lower sensitivity with less false-positives. The sensitivities of 81-100% with positive predictive values of 20-64% with the cervical length of 30-32 mm, are reported in a review that included 20 studies of cervical length measurement by transvaginal or transperineal ultrasonography in women admitted in threatened preterm labour3.
Cervical length measurement is superior to the fibronectin test in certain circumstances including ruptured membranes, vaginal bleeding, after vaginal examination and recent intercourse. A study using fibronectin concentration combined with cervical length measurement in patients with threatened preterm labour concluded that the fibronectin test does not improve the prediction when used together with cervical length measurements4. Transvaginal or transperineal scanning does however require a skilled clinician and a relatively high-resolution scanner.
TVS is shown to be well accepted by pregnant women5 and is safe as far as inoculation is concerned even with PPROM6;7. It is reproducible and inter-observer and intra-observer variability of TVS are both less than 10%8 compared with digital examination which has inter-observer variability of 52%9. TVS has an advantage over transabdominal sonographic evaluation as maternal habitus, position of the cervix and degree of bladder filling unlikely to affect visualisation of the cervix.
Transabdominal scanning provides essential information in preterm labour such as fetal presentation, as the incidence of malpresentation is higher in preterm deliveries, and estimated fetal weight – although there are limitations in its accuracy with advancing gestation or in the presence of anhydramnios and oligohyroamnios. The confirmation of fetal presentation is crucial as far as mode of delivery is concerned, and allows appropriate counselling for the mother. Estimated fetal weight particularly during a very preterm labour also allows paediatricians and obstetricians to counsel the parents accordingly and make joint plans as far as mode of delivery and neonatal resuscitation are concerned.
The absence of fetal breathing movements observed by ultrasound scanning is reported to be associated with preterm labour in patients admitted to labour ward with threatened preterm labour10. There are no randomised studies to suggest routine use in the diagnosis of preterm labour.
Unbooked pregnancies
A systematic review11 revealed that unbooked women had a significantly worse perinatal and maternal outcome. They were more likely to deliver preterm, have growth retarded babies and were at greater risk of a stillbirth or neonatal death. For women present in labour with no previous antenatal care, ultrasound scanning provides essential information including fetal presentation and placental position. Fetal biometry should be measured to help clinicians assess the correct gestational age if there is any doubt about this although sonographic biometry to date the pregnancy has limits in accuracy and may have a variation of 3 to 4 weeks especially at term. Similarly estimation of fetal weight may vary from actual fetal weight up to 10-15% more or less in the third trimester.
Induction of labour
Induction of labour occurs in 20% of pregnant women and 20% of these women end up with caesarean section12. Traditionally transvaginal examination for Bishops score is used to predict the success of induction to limit the number of failed inductions. Studies show that pre-induction cervical length measurement by TVS provides better prediction of successful induction of labour and mode of delivery than transvaginal examination which is subjective13 and has a poor predictive value for the outcome of induction14.
A recent meta-analysis included 20 trials with 3101 patients and showed that cervical length by TVS correlates with succe